1. Field of the Invention
The present invention relates to an optical modulating device, an optical transmitting apparatus using the optical modulating device, a method of controlling the optical modulating device, and a control program recording medium used for the control method, and in particular, to a control technique of stabilizing an optical signal outputted by a Mach Zehnder modulator (hereinafter simply referred to as an “MZ modulator”) used as a part of an electric/optical converter. This technique is used for, for example, a long-distance high-speed optical fiber communication network.
2. Description of the Related Art
Among conventional modulating methods for optical transmitting apparatuses in an optical communication system using high-speed optical fibers, there is a direct modulating method in which a semiconductor laser is driven by a digital input signal, thereby directly carrying out electric/optical conversion. However, as a bit rate of the digital input signal increases, it becomes difficult to achieve long-distance optical-fiber transmissions using the direct modulating method, owing to the adverse effects of variations (chirping) in the wavelength of an output optical signal or dispersion of the transmission light in optical fibers.
Thus, an MZ modulator, which is an external modulator, has been introduced. This, in principle, avoids variations in wavelength. Further, for an optical transmitting apparatus using the MZ modulator, transmitted outputs (optical signals) must be stabilized so as not to be affected by variations in temperature, age deterioration, and the like, so that an optical communication system can always operate stably.
FIG. 1 shows an example of the relationship (light transmittance) between a driving voltage and an output optical signal, the driving voltage varying according to the amplitude of an electric signal inputted to an MZ modulator. In this description, the input signal is a binary digital signal.
In FIG. 1, the difference between the driving voltage of the input electric signal obtained at the maximum value (peak) of the light transmittance and the driving voltage obtained at the minimum value (zero or null) of the light transmittance is defined as “Vπ”. The point at which the light transmittance has an intermediate value between the maximum and minimum values and at which the characteristic curve has a positive inclination is called as “QUAD point”. A driving voltage corresponding to the QUAD point is defined as “Vquad”.
Further, the driving voltages for the MZ modulator corresponding to logical values “0” and “1” of the input electric signal are defined as V0 and V1. An intermediate value (V0+V1)/2 is called a “bias voltage Vb (an operating point of the MZ modulator).
In FIG. 1, an optical transmission characteristic curve A indicates that the operating point of the MZ modulator is optimum (the relationship between the input electric signal and the optical transmission characteristic is optimum). An output optical signal obtained in this case is shown at A′.
Efficient optical modulation can be accomplished by thus driving the MZ modulator using the driving voltages V0 and V1, with which the light transmittance of the MZ modulator has its maximum and minimum values, respectively. It is thus possible to transmit an optical signal in which the ratio of the maximum transmittance to the minimum transmittance, i.e. an optical extinction ratio is high. In this case, the difference between V0 and V1 equals the Vπ. Further, the value Vb equals the value Vquad.
On the other hand, the optical transmission characteristic of the MZ modulator is subject to a change (operating point drift) called a “DC drift” due to variations in DC bias voltage, temperature, aging, etc. As a result, output optical signals may be degraded.
In FIG. 1, curves B and B′ indicate an optical transmission characteristic and an output optical signal observed if a DC drift occurs in an initial state indicated by curves A and A′. That is, the DC drift is a phenomenon in which the optical transmission characteristic is shifted in the direction of abscissa in FIG. 1.
If a DC drift occurs and the driving voltage then has the same value as that in its initial state, the waveform of the output optical signal B′ and its optical extinction ratio are degraded as shown in FIG. 1. This DC drift must be compensated. That is, if a DC drift occurs, it must be compensated by considering the magnitude of the drift to be the magnitude of a change in the driving voltage and then changing the values of the driving voltages V0 and V1 by the magnitude of the change in voltage ΔVb. This compensation can be equivalently carried out by changing the bias voltage Vb by ΔVb.
For example, Jpn. Pat. Appln. KOKAI Publication No. 3-251815 “Method of Controlling External Modulator” discloses a conventional control method of compensating for a DC drift in the MZ modulator to allow the modulator to operate stably. A control circuit for carrying out the method is configured, for example, as shown in FIG. 20.
According to the principle of this control method, a low-frequency superposing circuit 141 first superposes a low-frequency signal (normalized signal) transmitted by a low-frequency transmitter 147 and having a normalized frequency, on an input signal (modulates the amplitude of the input signal using the normalized signal). An output from the low-frequency superposing circuit 141 is then inputted to an MZ modulator 143 via a driving circuit 142. The low-frequency signal from the low-frequency oscillator 147 is also supplied to a low-frequency signal detecting circuit 145.
The MZ modulator 143 uses a signal provided by the driving circuit 142 to modulate light emitted by a semiconductor laser light source 144 so as to convert it into an optical modulated signal. The MZ modulator 143 then outputs the optical modulated signal to an optical transmission path 148. A part of the optical signal is branched and inputted to the low-frequency signal detecting circuit 145. A monitoring photodiode in the low-frequency signal detecting circuit 145 converts the inputted optical signal into an electric signal. This electric signal contains a low-frequency component of the normalized signal. This frequency component of the normalized signal has its phase vary through 180° depending on the direction of an operating point drift. By multiplying the signal containing this frequency component by the normalized signal from the low-frequency oscillator 147 and then carrying out synchronous detection, it is possible to detect a positive or negative DC component (an error signal) dependent on the direction of the operating point drift. Thus, the operating point of the MZ modulator 143 can be optimally retained by causing a control circuit 146 to control the operating point so as to zero the DC component. This drift compensating operation is characterized by its relatively high speed.
Without any operating point drifts, an optical signal outputted by the MZ modulator 143 has its amplitude modulated at a frequency double the normalized frequency. Thus, this signal does not contain any frequency components of the normalized signal. In this case, no DC components are detected.
However, with the above conventional control method, the MZ modulator 143 is driven by a driving signal modulated by superposing a low-frequency sinusoidal wave on a very high-frequency input signal. It is thus essential to have the driving circuit (variable gain amplifier) 142 that has a wide dynamic range enough to linearly vary gain up to the maximum amplitude of this driving signal. It is technically difficult to realize such a high-output gain and high-speed variable gain amplifier having a wide dynamic range. Such a variable gain amplifier is also expensive.
Further, in FIG. 1, only if the difference between the driving voltages V0 and V1 for the MZ modulator 143, which correspond to the logical values of an input signal, equals the difference Vπ between a driving voltage obtained at the maximum light transmittance and a driving voltage obtained at the minimum light transmittance (Vb equals Vquad, i.e. the operating point of the MZ modulator 143 is optimum), then a control operation is performed correctly.
As described above, a problem with the conventional control method for an MZ type optical modulator is that it requires an expensive variable gain amplifier having a wide dynamic range. Another problem is that a control operation is performed incorrectly if the difference between the driving voltages V0 and V1 for the MZ modulator does not equal Vπ.
Further, an optical communication system using high-speed optical fibers generally uses an NRZ (Non Return to Zero) modulating method of carrying out optical modulation using an NRZ signal that is a binary digital signal. In this case, if an attempt is made to increase signal transmission capacity using a time division multiplexing (TDM) method, transmission distance may be limited by degradation of the waveform of the transmission signal caused by the dispersion (GVD) of wavelengths in the transmitted optical signal. Further, dispersion tolerance is in inverse proportion to the square of a data bit rate. Accordingly, given that the dispersion tolerance is about 800 ps/nm in a 10-Gb/s system, it decreases down to 1/16-th, i.e. about 50 ps/nm in a system with a quadruple bit rate, i.e. a 40-Gb/s system. It is thus difficult to put this system to practical use.
An optical duo binary modulating method has been proposed as a method of reducing the degradation of the waveform caused by the wavelength dispersion. Refer to, for example, A. J. Price et al., “Reduced bandwidth Optical Digital Intensity Modulation with Improved Chromatic Dispersion Tolerance”, Electron. Lett., vol. 31, No. 1, pp. 58-59, 1995.
The optical duo binary modulating method reduces the bandwidth of an optical signal spectrum to about half to weaken the effects of the wavelength dispersion compared to the NRZ modulating method. For example, the bandwidth of an optical signal spectrum in a 10-Gb/s system has a frequency of 10 GHz and a wavelength of 0.1 nm with the NRZ modulating method. By contrast, it has a frequency of 5 GHz and a wavelength of 0.2 nm with the optical duo binary modulating method. That is, the optical duo binary modulating method reduces the bandwidth to half compared to the NRZ modulating method.
Light propagation speed varies depending on the wavelength. As the bandwidth of the optical signal spectrum increases, the magnitude of a variation in bit rate increases, which more markedly disrupts the waveform during long-distance transmissions. Thus, if the bandwidth of the optical signal spectrum can be reduced using the optical duo binary modulating method, the magnitude of a variation in bit rate decreases to enhance the dispersion tolerance.
FIG. 21 shows a configuration of a modulating section based on the conventional optical duo binary modulating method. The waveform diagram in FIG. 22 is provided in order to describe the optical duo binary modulating method.
In FIG. 21, reference numeral 151 denotes a semiconductor laser, and reference numeral 152 denotes an MZ type modulator. Reference numeral 153 denotes a precoder that encodes a binary NRZ input signal. Reference numeral 154 denotes a modulator driver that functions as an amplitude adjusting section. Reference numeral 155 denotes a low pass filter (LPF) having a pass band for a low frequency signal with a frequency that is about quarter a bit rate (BR). Reference numeral 156 denotes a bias adjusting circuit (bias T), and 157 denotes a terminator.
After being encoded by the precoder 153, a binary NRZ signal input has its amplitude adjusted by the modulator driver 154. The adjusted signal passes through the low pass filter 155 and is thus converted into a ternary signal. The converted signal is applied to a signal electrode of the MZ type modulator 152.
As shown in FIG. 22, the optical duo binary modulating method doubles the driving voltage Vb for the MZ type modulator 152 compared to the NRZ modulating method. Consequently, the MZ type modulator 152 is modulated at a driving amplitude (Vpp=2Vπ) double that of Vπ. Further, a DC bias voltage (the center of the driving voltage) is set so that the modulator is driven between two adjacent ones P1 and P2 of periodic light emission peaks on a characteristic curve for driving voltage vs. optical output.
Now, operations of the circuit in FIG. 21 will be described with reference to FIGS. 19A to 19H.
FIGS. 19A and 19B show a binary NRZ input signal and its eye pattern. FIGS. 19C and 19D show an output signal from the precoder 153 and its eye pattern. FIGS. 19E and 19F show an output signal from the low pass filter 155 and its eye pattern. FIGS. 19G and 19H show an output optical signal from the MZ type modulator 152 and its eye pattern.
A comparison of FIG. 19A with FIG. 19G indicates the output optical signal from the MZ type modulator has exactly the same logic as the binary NRZ signal input. Accordingly, a receiver (not shown) that receives this optical signal can convert it into a binary NRZ signal without using any decoders.
The above optical duo binary modulating method is characterized by reducing the bandwidth of an optical signal spectrum to about half compared to the conventional NRZ modulating method. It can thus weaken the adverse effects of the wavelength dispersion to allow channels to be more densely arranged using a wavelength dispersion multiplexing (WDM) method. That is, if an attempt is made to increase the signal transmission capacity using the wavelength dispersion multiplexing technique, the wavelength band that can be amplified by an optical amplifier is a limiting factor. However, the channels can be more densely arranged within an amplification band of the optical amplifier by utilizing the narrow-band characteristic of an optical signal spectrum obtained by the optical duo binary modulating method.
As described above, the conventional optical duo binary modulating method using the MZ type modulator is disadvantageous in that an output optical signal is unstable owing to variations in the characteristics of the MZ type modulator. It is thus necessary to control the bias voltage in response to a variation in operating point so that an optical communication system based on the optical duo binary modulating method using the MZ type modulator always operates stably to stabilize transmission outputs (optical signals).